The S822 and S823 Airfoils - NWTC Information Portal

37 Pages·2004·1.22 MB·English

January 2005 • NREL/SR-500-36342
The S822 and S823 Airfoils

October 1992—December 1993
D.M. Somers
Airfoils, Inc.
State College, Pennsylvania
National Renewable Energy Laboratory
1617 Cole Boulevard, Golden, Colorado 80401-3393
303-275-3000 • www.nrel.gov
Operated for the U.S. Department of Energy
Office of Energy Efficiency and Renewable Energy
by Midwest Research Institute • Battelle
Contract No. DE-AC36-99-GO10337
January 2005 • NREL/SR-500-36342
The S822 and S823 Airfoils

October 1992—December 1993
D.M. Somers
Airfoils, Inc.
State College, Pennsylvania
NREL Technical Monitor: Jim Tangler

Prepared under Subcontract No. AAO-3-13023-01-104879
National Renewable Energy Laboratory
1617 Cole Boulevard, Golden, Colorado 80401-3393
303-275-3000 • www.nrel.gov
Operated for the U.S. Department of Energy
Office of Energy Efficiency and Renewable Energy
by Midwest Research Institute • Battelle
Contract No. DE-AC36-99-GO10337
This publication was reproduced from the best available copy
submitted by the subcontractor and received no editorial review at NREL
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Table of Contents

Abstract................................................................................................................................1
Introduction..........................................................................................................................1
Symbols................................................................................................................................1
Airfoil Design......................................................................................................................2
Objectives and Constraints......................................................................................2
Philosophy................................................................................................................3
Execution.................................................................................................................5
Discussion of Results...........................................................................................................6
S822 Airfoil.............................................................................................................6
S823Airfoil..............................................................................................................7
Concluding Remarks............................................................................................................8
References............................................................................................................................9

List of Tables

Table I. Airfoil Design Specifications...........................................................................10
Table II. S822 Airfoil Coordinates.................................................................................11
Table III. S826 Airfoil Coordinates.................................................................................12

List of Figures

Figure 1: Airfoil shapes ..................................................................................................13
Figure 2: Inviscid pressure distributions for S822 airfoil........................................14 – 17
Figure 3: Section characteristics of S822 airfoil with transition free,
transition fixed, and rough.......................................................................18 – 22
Figure 4: Inviscid pressure distributions for S823 airfoil........................................23 – 27
Figure 5: Section characteristics of S823 airfoil and transition free,
transition fixed, and rough.......................................................................28 – 32

iii
THE S322 AND S823 AIRFOILS
Dan M. Somers
December 1993
ABSTRACT
A family of thick airfoils for 3- to 10-meter, stall-regulated, horizontal-axis wind turbines,
the S822 and S823, has been designed and analyzed theoretically. The primary objectives of
restrained maximum lift, insensitive to roughness, and low profile drag have been achieved. The
constraints on the pitching moments and airfoil thicknesses have been satisfied.
INTRODUCTION
The family of thick airfoils designed under this study is intended for 3- to 10-meter, stall-
regulated, horizontal-axis wind turbines. Four earlier thick-airfoil families, the S809, S 8 10, and
S811 (ref. l), the S812, S813,5814, and S815 (refs. t and 2), the S816, S817, and S818 (ref. 3),
and the S819, S820, and 5821 (ref. 4), were designed for 20- to 30-meter, 20- to 30-meter, 30- to
40-meter, and 10- to 20-meter wind turbines, respectively.
The specific tasks performed under this study are described in National Renewable Energy
Laboratory (NREL) Subcontract Number AAO-3- 13023-01-1 04879. The specifications for the
airfoils are outlined in the Statement of Work. These specifications were later refined during
telephone conversations with Mr. James L. Tangler of NREL.
Because of the limitations of the theoretical methods (refs. 5 and 6) employed in this study,
the results presented are in no way guaranteed to be accurate-either in an absolute or in a relative
sense. This statement applies to the entire study,
SYMBOLS
pressure coefficient
CP
C airfoil chord, meters
cd section profile-drag coefficient
C1 section lift coefficient
section pitching-moment coefficient about quarter-chord point
Cm
L. lower surface
MU boundary-layer transition mode (ref. 6)
R Reynolds number based on ftee-stream conditions and airfoil chord
S. boundary-layer separation location, 1 - ssep/c
arc length along which boundary layer is separated, meters
Ssep
arc length along which boundary layer is turbulent including ssepm, eters
Stub
T. boundary-layer transition location, 1 - St,b/C
U. upper surface
airfoil abscissa, meters
X
Y airfoil ordinate, meters
a angle of attack relative to chord line, degrees
AIRFOIL DESIGN
OBJECTIVES AND CONSTRAINTS
The design specifications for the family of airfoils are contained in table I. The family
consists of two airfoils, tip and root, corresponding to the 0.90 and 0.40 blade radial stations,
.
respectively
Two primary objectives are evident from the specifications. The first objective is to re-
strain the maximum lift coefficient of the tip airfoil to the relatively low value of 1. OO. In contrast,
the maximum lift coefficient of the root airfoil should be as high as possible. A requirement
related to this objective is that the maximum lift coefficient not decrease with transition fixed near
the leading edge on both surfaces, The second objective is to obtain low profile-drag coefficients
2
over the ranges of lift coefficients from 0.2 to 0.8 for the tip airfoil and from 0.4 to 1.0 for the root
airfoil.
Two major constraints were placed on the designs of these airfoils. First, the zero-lift
pitching-moment coefficients must be no more negative than 4 0 7f or the tip airfoil and -0.15 for
the root airfoil. Second, the airfoil thicknesses must equal 16-percent chord for the tip airfoil and
21-percent chord for the root airfoil.
PHILOSOPHY
Given the above objectives and constraints, certain characteristics of the designs are
evident. The following sketch illustrates a drag polar which meets the goals for these designs.
‘d
Sketch 1
The desired airfoil shapes can be traced to the pressure distributions which occur at the various
points in sketch 1. Point A is the lower limit of the low-drag, lift-coefficient range. The lift
coefficient at point A is 0.1 lower than the objective specified in table I. The difference is intended
as a margin against such contingencies as manufacturing tolerances, operational deviations, three-
dimensional effects, and inaccuracies in the theoretical method. A similar margin is also desirable
at the upper limit of the low-drag, lift-coefficient range, point B. The drag at point B is not as low
as at point A, unlike the polars of many other laminar-flow airfoils where the drag within the
laminar bucket is nearly constant. This characteristic is related to the elimination of significant
(drag-producing) laminar separation bubbles on the upper surface (see ref. 7) and is acceptable
because the ratio of the profile drag to the total drag of the wind-turbine blade decreases with
increasing lift coefficient. The drag increases very rapidly outside the laminar bucket because the
boundary-layer transition point moves quickly toward the leading edge. This feature results in a
3
rather sharp leading edge which produces a suction peak at higher lift coefficients, which limits the
maximum lift coefficient and ensures that transition on the upper surface will occur very near the
leading edge. Thus, the maximum lift coefficient occurs with turbulent flow along the entire upper
surface and, therefore, should be insensitive to roughness at the leading edge. Point C is the
maximum lift coefficient.
From the preceding discussion, the pressure distributions along the polar can be deduced.
The pressure distribution at point A for the tip airfoil should look something like sketch 2. (The
pressure distribution for the root airfoil should be qualitatively similar.)
U.
0I I I 1 I 0.1 5 I 1 x/c 1 I 11
Sketch 2
To achieve low drag, a favorable pressure gradient is desirable along the upper surface to about
50-percent chord. Aft of this point, a short region having a shallow, adverse pressure gradient
(“transition ramp”) promotes the efficient transition from laminar to turbulent flow (ref. 8). This
short region is followed by a steeper, nearly linear pressure recovery. The specific pressure re-
covery employed represents a compromise among maximum lift, low drag, and docile stall
characteristics. The steep adverse pressure gradient on the upper surface aft of about 95-percent
chord is a ‘separation ramp,’ originally proposed by F. X. Wortmann, which confines turbulent
separation to a small region near the trailing edge. By controlling the movement of the separation
point at high angles of attack, high lift coefficients can be achieved with little drag penalty, This
feature has the added benefit that it promotes docile stall characteristics. (See ref. 9.)
A mildly adverse pressure gradient along the lower surface to about 55-percent chord is
able to sustain laminar flow and, therefore, low drag for the relatively low design Reynolds
number of 0.6 x lo6. This region is followed by a curved transition ramp (ref. 7) similar to the one
on the upper surface. The transition ramp is followed by a nearly linear pressure recovery.
The amounts of pressure recovery on the two surfaces are determined by the airfoil-
thickness and pi tching-mornent const rain ts.
4
At point Byt he pressure distribution should look like sketch 3.
Sketch 3
No suction spike exists at the leading edge. Instead, the peak occurs just aft of the leading edge.
This feature results from incorporating increasingly favorable pressure gradients toward the lead-
ing edge. It allows a wider laminar bucket to be achieved and higher lift coefficients to be reached
without significant separation.
EXECUTION
Given the pressure distributions previously discussed, the design of the airfoils is reduced
to the inverse problem of transforming the pressure distributions into airfoil shapes. The Eppler
Airfoil Design and Analysis Code (refs. 5 and 6) was used because of con€idence gained during
the design, analysis, and experimental verification of several other airfoils. (See refs. 10-12.)
The tip airfoil is designated the S822. The root airfoil, the 5823, was derived from the
S822 airfoil to increase the aerodynamic and geometric compatibilities of the two airfoils. The
airfoil shapes are shown in figure 1 and the coordinates are contained in tables I1 and 111. The S822
airfoil thickness is 16-percent chord and the S823,2Lpercent chord.
5
DISCUSSION OF RESULTS
S 822 AIRFOIL
Pressure Distributions
The inviscid (potential-flow) pressure distributions for the S 822 airfoil for various angles
of attack are shown in figure 2. Because the free-stream Mach number for all relevant operating
conditions remains below 0.2, these and all subsequent results are incompressible.
Transition and Separation Locations
The variation of boundary-layer transition location with lift coefficient for the S822 airfoil
is shown in figure 3. It should be remembered that the method of references 5 and 6 ‘defines’ the
transition location as the end of the laminar boundary layer whether due to natural transition or
laminar separation. Thus, for conditions which result in relatively long laminar separation bubbles
(low lift coefficients for the upper surface and high lift coefficients for the lower surface and/or
low Reynolds numbers), poor agreement between the predicted ‘transition? locations and the lo-
cations measured experimentally can be expected. This poor agreement is worsened by the fact
that transition is normally confirmed in the wind tunnel only by the detection of attached turbulent
flow. For conditions which result in shorter laminar separation bubbles (high lift coefficients for
the upper surface and low lift coefficients for the lower surface and/or high Reynolds numbers),
the agreement between theory and experiment should be quite good. (See ref. 13.)
The variation of turbulent boundary-layer separation location with lift coefficient for the
5822 airfoil is shown in figure 3. A small separation is predicted on the upper surface at all lift
Coefficients. This separation, which is caused by the separation ramp (fig. 2), increases in length
with transition fixed near the leading edge. A small separation is predicted on the lower surface at
lift coefficients below about 0.2 with transition fixed. This separation is not considered important
because it occurs at lift coefficients which are not typical of normal wind-turbine operations. Also,
such separation usually has little effect on the section characteristics. (See ref. 13.)
Section Characteristics
-Rey nolds number effects.- The section characteristics of the S822 airfoil are shown in
figure 3. It should be noted that the maximum lift coefficient predicted by the method of refer-
ences 5 and 6 is not always realistic. Accordingly, an empirical criterion should be applied to the
computed results. This criterion assumes that the maximum lift coefficient has been reached if the
drag coefficient of the upper surface is greater than 0.0240 or if the length of turbulent separation
along the upper surface is greater than 0.10. Thus, the maximum lift coefficient for the design
6